1. Field of the Invention
The present invention relates to a polishing apparatus and a polishing method, and more particularly to a polishing apparatus and a polishing method for polishing and planarizing a substrate such as a semiconductor wafer on which a conductive film such as a copper (Cu) layer or a tungsten (W) layer is formed. Further, the present invention relates to a program for measuring a film thickness of a substrate when the substrate is polished by such polishing apparatus and such polishing method.
2. Description of the Related Art
In order to form interconnect circuits on a semiconductor substrate, there has been known a process in which copper plating is performed to form a plated copper layer and an unnecessary portion of the plated copper layer thus formed is removed by chemical mechanical polishing (CMP) to form a copper interconnect layer. In this chemical mechanical polishing, it is necessary that the progress of polishing of a conductive film such as a copper layer should be exactly grasped and an end point of the polishing should be exactly detected. In order to detect such end point of the polishing, there has been known a method of measuring a film thickness of a conductive film using an optical sensor or a method of measuring a film thickness of a conductive film using an eddy current sensor for measuring a film thickness from a magnitude of the eddy current generated in a conductive film (for example Japanese Laid-open Patent Publication No. 2005-11977).
The eddy current sensor uses eddy current generated in a conductive film such as a metal film formed in a top layer of a semiconductor wafer to measure a film thickness of the conductive film. Specifically, a magnetic flux is formed by a sensor coil, and the magnetic flux passes through the conductive film of the semiconductor wafer located in front of the sensor coil, thus being alternatively changed. Thus, the eddy current is generated in the conductive film, and the eddy current flows in the conductive film to cause eddy current loss. In the eddy current sensor, the semiconductor wafer and the conductive film can be regarded as an equivalent circuit and the thickness of the conductive film on the semiconductor wafer can be measured by measuring the eddy current loss.
The film thickness to be measured by the eddy current sensor is a film thickness of a conductive film as the uppermost layer. However, the magnetic flux of the eddy current sensor is not limited only to the uppermost layer, and if a layer or layers that underlie the uppermost layer have conductivity, measurements by the eddy current sensor are affected by an underlayer or underlayers. Further, recently, interconnect layers formed by an interconnect forming process become high density and are multilayered, and the upper layer tends to have an interconnect width wider than an interconnect width of the lower layer and an interconnect thickness thicker than an interconnect thickness of the lower layer. Therefore, as the number of laminations of interconnect circuits increases, output signals from the eddy current sensor are more highly affected by the underlayer or underlayers. The output signals which have been affected by the underlayer or underlayers do not reflect polishing conditions exactly, and thus detection of an end point of the polishing becomes unstable. Therefore, there has been developed a method in which a semiconductor wafer is divided into a plurality of zones and an end point of the polishing is detected on the basis of features of signals obtained from the respective zones.
An interconnect forming process of the semiconductor wafer is normally carried out by forming a plurality of dies (part in which electronic circuits are formed) on a single wafer. In general, a conductive material such as a metal for interconnect formation is not formed between the adjacent dies. Therefore, in the case where the stage of lamination progresses, the signal wave form of the eddy current sensor at a measurement point on the die is quite different from signal wave form of the eddy current sensor at a measurement point located between the adjacent dies. Since the semiconductor wafer is rotated during polishing, even if the same zone is measured, the proportion of the dies in the zone is changed for each measurement. As a result, exact data cannot be obtained. In order to reduce such influence, there has been developed a method in which data obtained by an eddy current sensor are smoothed over an entire surface of a semiconductor wafer to detect an end point of polishing, without division of zones.
As described above, when the end point of the polishing is being detected, it is difficult to measure the film thickness stably by the influence of noise in the interconnect layer, during polishing, or the influence of the interconnect pattern of the underlying layer. Further, it is difficult to obtain information about the film thickness by smoothing signals from the sensor over the entire surface of the wafer. Even if the end point of the polishing is detected from data regarding the film thickness which have been affected by such noise or such interconnect pattern of the underlying layer, the end point of the polishing cannot be detected stably.